package com.glgjing.gifencoder;

import java.io.IOException;
import java.io.OutputStream;


public class LZWEncoder {

  private static final int EOF = -1;
  private static final int BITS = 12;
  private static final int HSIZE = 5003; // 80% occupancy

  private int imgW, imgH;
  private int initCodeSize;
  private int remaining;
  private int curPixel;
  private int n_bits; // number of bits/code
  private int maxbits = BITS; // user settable max # bits/code
  private int maxcode; // maximum code, given n_bits
  private int maxmaxcode = 1 << BITS; // should NEVER generate this code
  private int hsize = HSIZE; // for dynamic table sizing
  private int free_ent = 0; // first unused entry
  private int g_init_bits;
  private int ClearCode;
  private int EOFCode;
  private int cur_accum = 0;
  private int cur_bits = 0;
  private int a_count;

  private int[] htab = new int[HSIZE];
  private int[] codetab = new int[HSIZE];
  private int masks[] = {
      0x0000,
      0x0001,
      0x0003,
      0x0007,
      0x000F,
      0x001F,
      0x003F,
      0x007F,
      0x00FF,
      0x01FF,
      0x03FF,
      0x07FF,
      0x0FFF,
      0x1FFF,
      0x3FFF,
      0x7FFF,
      0xFFFF};

  private byte[] pixAry;
  private byte[] accum = new byte[256];
  private boolean clear_flg = false;


  LZWEncoder(int width, int height, byte[] pixels, int color_depth) {
    imgW = width;
    imgH = height;
    pixAry = pixels;
    initCodeSize = Math.max(2, color_depth);
  }

  // Add a character to the end of the current packet, and if it is 254
  // characters, flush the packet to disk.
  void char_out(byte c, OutputStream outs) throws IOException {
    accum[a_count++] = c;
    if (a_count >= 254)
      flush_char(outs);
  }

  // table clear for block compress
  void cl_block(OutputStream outs) throws IOException {
    cl_hash(hsize);
    free_ent = ClearCode + 2;
    clear_flg = true;

    output(ClearCode, outs);
  }

  // reset code table
  void cl_hash(int hsize) {
    for (int i = 0; i < hsize; ++i)
      htab[i] = -1;
  }

  void compress(int init_bits, OutputStream outs) throws IOException {
    int fcode;
    int i /* = 0 */;
    int c;
    int ent;
    int disp;
    int hsize_reg;
    int hshift;

    // Set up the globals:  g_init_bits - initial number of bits
    g_init_bits = init_bits;

    // Set up the necessary values
    clear_flg = false;
    n_bits = g_init_bits;
    maxcode = MAXCODE(n_bits);

    ClearCode = 1 << (init_bits - 1);
    EOFCode = ClearCode + 1;
    free_ent = ClearCode + 2;

    a_count = 0; // clear packet

    ent = nextPixel();

    hshift = 0;
    for (fcode = hsize; fcode < 65536; fcode *= 2)
      ++hshift;
    hshift = 8 - hshift; // set hash code range bound

    hsize_reg = hsize;
    cl_hash(hsize_reg); // clear hash table

    output(ClearCode, outs);

    outer_loop:
    while ((c = nextPixel()) != EOF) {
      fcode = (c << maxbits) + ent;
      i = (c << hshift) ^ ent; // xor hashing

      if (htab[i] == fcode) {
        ent = codetab[i];
        continue;
      } else if (htab[i] >= 0) // non-empty slot
      {
        disp = hsize_reg - i; // secondary hash (after G. Knott)
        if (i == 0)
          disp = 1;
        do {
          if ((i -= disp) < 0)
            i += hsize_reg;

          if (htab[i] == fcode) {
            ent = codetab[i];
            continue outer_loop;
          }
        } while (htab[i] >= 0);
      }
      output(ent, outs);
      ent = c;
      if (free_ent < maxmaxcode) {
        codetab[i] = free_ent++; // code -> hashtable
        htab[i] = fcode;
      } else
        cl_block(outs);
    }
    // Put out the final code.
    output(ent, outs);
    output(EOFCode, outs);
  }

  void encode(OutputStream os) throws IOException {
    os.write(initCodeSize); // write "initial code size" byte

    remaining = imgW * imgH; // reset navigation variables
    curPixel = 0;

    compress(initCodeSize + 1, os); // compress and write the pixel data

    os.write(0); // write block terminator
  }

  // Flush the packet to disk, and reset the accumulator
  void flush_char(OutputStream outs) throws IOException {
    if (a_count > 0) {
      outs.write(a_count);
      outs.write(accum, 0, a_count);
      a_count = 0;
    }
  }

  final int MAXCODE(int n_bits) {
    return (1 << n_bits) - 1;
  }

  private int nextPixel() {
    if (remaining == 0)
      return EOF;

    --remaining;

    byte pix = pixAry[curPixel++];

    return pix & 0xff;
  }

  void output(int code, OutputStream outs) throws IOException {
    cur_accum &= masks[cur_bits];

    if (cur_bits > 0)
      cur_accum |= (code << cur_bits);
    else
      cur_accum = code;

    cur_bits += n_bits;

    while (cur_bits >= 8) {
      char_out((byte) (cur_accum & 0xff), outs);
      cur_accum >>= 8;
      cur_bits -= 8;
    }

    // If the next entry is going to be too big for the code size,
    // then increase it, if possible.
    if (free_ent > maxcode || clear_flg) {
      if (clear_flg) {
        maxcode = MAXCODE(n_bits = g_init_bits);
        clear_flg = false;
      } else {
        ++n_bits;
        if (n_bits == maxbits)
          maxcode = maxmaxcode;
        else
          maxcode = MAXCODE(n_bits);
      }
    }

    if (code == EOFCode) {
      // At EOF, write the rest of the buffer.
      while (cur_bits > 0) {
        char_out((byte) (cur_accum & 0xff), outs);
        cur_accum >>= 8;
        cur_bits -= 8;
      }

      flush_char(outs);
    }
  }
}
